September 27, 1889.]- 



SCIENCE. 



219 



elusions to be drawn, unless the characters of such media have 

 been thoroughly well tested, and the light they transmit has been 

 measured. An impure spectrum is even more dangerous to rely 

 upon, since the access of white light would be sure to vitiate the 

 results. 



Perhaps one of the most puzzling phenomena to be met with in 

 photography is the fact that the range of photographic action is 

 spread over so large a portion of the spectrum. The same diffi- 

 culty, of course, is felt in the matter of absorption, since the one is 

 dependent on the other. Absorption by a body we are accustomed, 

 and indeed obliged by the law of the conservation of energy, to 

 consider as due to the transferrence of the energy of the ether 

 wave-motion to the molecules and atoms comprising the body by 

 increasing the vibrations of one or both. 



In the case where chemical action takes place, we can scarcely 

 doubt that it is the atoms which in a great measure take up the 

 energy of the radiation falling on them, as chemical action is de- 

 pendent on the liberation of one or more atoms from the molecule ; 

 while, when the swings of the molecules are increased in amplitude, 

 we have a rise in temperature of the body. I shall confine the few 

 remarks I shall make on this subject to the case of chemical ac- 

 tion. The molecule of a silver salt, such as bromide of silver, 

 chemists are wont to look upon as composed of a limited and equal 

 number of atoms to form the molecule. When we place a thin 

 slab of this material before the slit of the spectroscope, we find a 

 total absorption in the violet and ultra-violet of the spectrum, and 

 a partial absorption in the blue and green, and a diminishing ab- 

 sorption in the yellow and red. A photographic plate containing 

 this same salt is acted upon in exactly the same localities and in 

 the same relative degree as where the absorption takes place. 

 Here, then, we have an example of, it may be, the vibrations of four 

 atoms, one of which at least is isochronous, or partially so, with the 

 waves composing a large part of the visible spectrum. The ex- 

 planation of this is somewhat obscure. A mental picture, how- 

 ever, may help us. If we consider that, owing to the body acted 

 upon being a solid, the oscillations of the molecules and atoms are 

 confined to a limited space, it probably happens that between the 

 times hi which the atoms occupy, in regard to one another, the 

 same relative positions, the component vibrations of, say, two of the 

 atoms vary considerably in period. An example of what I mean is 

 found in a pendulum formed of a bob and an elastic rod. If the 

 bob be made to vibrate in the usual manner, and at the same time 

 the elastic rod be elongated, it is manifest that we have a pendulum 

 of ever-varying length. At each instant of time the period of vi- 

 bration would differ from that at the next instant, if the oscillations 

 were completed. It is manifest that increased amplitude would be 

 given to the pendulum-swings by a series of well-timed blows dif- 

 fering very largely in period. At the same time there would be 

 positions of the pendulum in which some one series of well-timed 

 blows would produce the greatest effect. In a somewhat similar 

 manner we should imagine that the ethereal waves should produce 

 increased amplitude in the swing of the atoms between very wide 

 limits of period, and, further, that there should be one or more 

 positions in the spectrum when a maximum effect is produced. I 

 would here remark that the shape of the curves of sensitiveness, 

 when plotted graphically, of the different salts of silver to the spec- 

 trum, have a marked resemblance to the graphically drawn curves 

 of the three color-sensations of the normal eye, as determined by 

 Clerk Maxwell. May not the reason for the form of the one be 

 equally applicable for the other ? I only throw this out as evidence, 

 not conclusive indeed, that the color-sensitiveness of the eye is more 

 probably due to a photographic action on the sensitive retina than 

 to a merely mechanical action. That this is the case, I need 

 scarcely say has several times been propounded before. 



The ease with which a silver salt is decomposed is largely, if not 

 quite, dependent on the presence of some body which will take up 

 some of the atoms which are thrown off from it : for instance, in 

 chloride of silver we have a beautiful example of the necessity of 

 such a body. In the ordinary atmosphere the chloride is, of course, 

 colored by the action of light ; but if it be carefully dried and 

 purified, and placed in a good vacuum, it will remain uncolored for 

 years in the strongest sunlight. In this case the absence of air and 

 moisture is sufficient to prevent it discoloring. 



If in the vacuum, however, a drop of mercury be introduced, the 

 coloration by light is set up. We have the chlorine liberated from 

 the silver and combining with the mercury vapor, and a minute 

 film of calomel formed on the sides of the vessel. 



Delicate experiments show that not only is this absorbent almost 

 necessary when the action of light is so strong or so prolonged that 

 its effect is visible, but also when the exposure or intensity is so 

 small that the effect is invisible and only to be found by develop- 

 ment. The reason for this absorbent is not far to seek. If, for 

 instance, silver chloride be exposed to light in vacuo, although the 

 chlorine atoms may be swung off from the original molecule, yet 

 they may only be swung off to a neighboring molecule which has 

 lost one of its chlorine atoms, and an interchange of atoms merely 

 takes place. If, however, a chlorine absorbent be present which 

 has a greater affinity for chlorine than has the silver chloride which 

 has lost one of its atoms, then we may consider that the chlorine 

 atoms will be on the average more absorbed by the absorbent than 

 by the subchloride molecules. The distribution of the swung-off 

 atoms between the absorbent and the subchloride will doubtless be 

 directly proportional to their respective affinities for chlorine, and 

 so for the other salts of silver. If this be so, then it will be seen 

 that the greater the affinity of the absorbent for the halogen, the 

 more rapid will be the decomposition of the silver salt. This, then, 

 points to the fact that if any increase in the sensitiveness of a silver 

 salt is desired,' it will probably be brought about by mixing with it 

 some stronger halogen absorbent than has yet been done. 



The question as to what is the exact product of the decomposition 

 of a silver salt by the action of light is one which has not as yet 

 been fully answered. For my own part, I have my strong beliefs 

 and my disbeliefs. I fully believe the first action of light to be a 

 very simple one, though this simple action is masked by other ac- 

 tions taking place, due to the surroundings in which it takes place. 

 The elimination of one atom from a molecule of a silver salt leaves 

 the molecule in an unsatisfied condition, and capable of taking up 

 some fresh atom. It is this capacity which seemingly shrouds the 

 first action of light, since when exposure is prolonged the molecules 

 take up atoms of oxygen from the air or from the moisture in it. 

 Carey Lea of Philadelphia has within the last three years given 

 some interesting experiments on the composition of what he calls 

 the photochloride of silver, which is the chloride colored by light, 

 and Professor Hodgkinson has also taken up the matter. The 

 conclusions the former has drawn are, to my mind, scarcely yet to 

 be accepted. According to the latter experimentalist, the action of 

 light on silver chloride is to form an oxidized subsalt. This can 

 hardly be the case, except under certain conditions, since a colored 

 compound is obtained when the silver chloride is exposed in a 

 liquid in which there is no oxygen present. 



This coloration by light of the chloride of silver naturally leads 

 our thoughts to the subject of photography in natural colors. The 

 question is often asked when photography in natural colors will be 

 discovered. Photography in natural colors not only has been dis- 

 covered, but pictures in natural colors have been produced. I am 

 not alluding to the pictures produced by manual work, and which 

 have from time to time been foisted on a credulous public as being 

 produced by the action of light itself, much to the damage of pho- 

 tography, and usually of the so-called inventors. Roughly speak- 

 ing, the method of producing the spectrum in its natural colors is 

 to chlorinize a silver plate, expose it to white light till it assumes a 

 violet color, heat till it becomes rather ruddy, and expose it to a 

 bright spectrum. The spectrum colors are then impressed in their 

 natural tints. Experiment has shown that these colors are due to 

 an oxidized product being formed at the red end of the spectrum, 

 and a reduced product at the violet end. Photography in natural 

 colors, however, is only interesting from, a scientific point of view, 

 and, so far as 1 can see, can never have a commercial value. A 

 process, to be useful, must be one by which reproductions are 

 quickly made : in other words, it must be a developing and not a 

 printing process, and it must be taken in the camera ; for any 

 printing process requires not only a bright light, but also a pro- 

 longed exposure. Now, it can be conceived that in a substance 

 which absorbs all the visible spectrum the molecules can be so 

 shaken and sifted by the different rays, that eventually they sort 

 themselves into masses which reflect the particular rays by which 



